The present invention relates to a spin-injection magnetoresistance effect element.
Magnetic random access memory (MRAM) has been known as a non-volatile magnetic memory element of high speed, high integration, and low power consumption (refer to U.S. Pat. No. 6,081,445, for example). The MRAM employing a tunnel magnetoresistance (TMR) effect includes tunnel magnetoresistive elements that are coupled to selection transistors formed of MOS-FETs. In addition, bit lines are provided above the tunnel magnetoresistive elements, and word lines extending perpendicularly to the bit lines are provided below the tunnel magnetoresistive elements. The tunnel magnetoresistive element has a multi-layer structure including from its bottom an antiferromagnetic layer, a magnetization fixed layer (referred to also as a pinned layer), a tunnel insulating film, and a recording layer (referred to also as a free layer) of which magnetization direction is readily rotated. In writing of information (data) to this MRAM, a current of the positive or negative direction is applied to the bit line while a current of a constant direction is applied to the writing word line. The synthetic magnetic field arising from these currents changes the magnetization direction of the recording layer, to thereby record a logical value of “1” or “0” in the recording layer.
In this MRAM employing the TMR effect, magnetization reversal is caused by a current magnetic field. However, the current amount required for this magnetization reversal increases in almost inverse proportion to the size of the tunnel magnetoresistive element. Therefore, construction of a high-capacity non-volatile magnetic memory element array involves a problem of large current consumption.
Spin-injection magnetization reversal is a novel magnetization reversal (magnetic information writing) method that does not depend on a current magnetic field and offers reduced power consumption (refer to U.S. Pat. No. 6,714,444, for example). In the spin-injection magnetization reversal, the current amount required for magnetization reversal decreases as the size of a magnetic element such as a non-volatile magnetic memory element becomes smaller. Therefore, the spin-injection magnetization reversal is suitable for achieving a high-capacity (e.g. giga-bit class) non-volatile magnetic memory element array.
FIG. 11A is a conceptual diagram of a conventional non-volatile magnetic memory element that employs spin-injection magnetization reversal. The non-volatile magnetic memory element includes a magnetoresistance effect multi-layer film having a giant magnetoresistance (GMR) effect or a TMR effect. This multi-layer film is interposed between two electrodes 401 and 405. Specifically, this multi-layer film is formed of a magnetization reversal layer (referred to also as a free layer) 404 having a function of recording information, a magnetization reference layer (referred to also as a pinned layer) 402 having a fixed magnetization direction and thus serving as a spin filter, and a nonmagnetic film 403 interposed between the layers 404 and 402. A current flows perpendicularly to the planes of these films (see FIG. 11A). FIG. 11B schematically shows the planar shape of the magnetization reversal layer 404. The size of the magnetization reversal layer 404 is typically about 200 nm or smaller in order to achieve single-domain magnetization and reduce the critical current Ic of spin-injection magnetization reversal, although depending on the magnetic material and thickness of the layer 404. The magnetization reversal layer 404 can be magnetized in two or more directions (two lateral directions indicated by the arrowheads in FIG. 11A, for example) due to its adequate magnetic anisotropy. Each magnetization direction corresponds to recorded information. In the example of FIG. 11B, the magnetization reversal layer 404 is designed to have an elliptic planar shape so as to be provided with a shape magnetic anisotropy. The magnetization direction of the magnetization reference layer 402 is pinned typically by exchange coupling between the layer 402 and an antiferromagnetic layer 406 (see FIG. 11C). Additionally, a double spin filter structure is also known (see FIG. 11D). In this structure, in order to enhance the efficiency of spin-injection magnetization reversal, magnetization reference layers 402A and 402B are provided above and below the magnetization reversal layer 404 with the intermediary of nonmagnetic films 403A and 403B therebetween, respectively. This structure also includes antiferromagnetic layers 406A and 406B. In the examples of FIGS. 11A, 11C and 11D, sometimes the magnetization reversal layer 404 and the magnetization reference layer 402 (one of two magnetization reference layers 402A and 402B) are formed of a synthetic ferri-magnetic structure. The nonmagnetic films 403, 403A and 403B are composed of a metal material or an insulating material. In the examples of FIGS. 11A and 11C, sometimes another structure is also employed in which the magnetization reference layer 402 is sufficiently larger than the magnetization reversal layer 404 in order to suppress a leakage magnetic field from the layer 402 to the layer 404, in other words, in order to prevent the layers 402 and 404 from being magnetostatically coupled. In any structure, the conventional non-volatile magnetic memory element based on spin-injection magnetization reversal has a two-terminal spin transfer element structure in which a magnetoresistance effect multi-layer film is interposed between two electrodes.
In a high-capacity non-volatile magnetic memory element array of the giga-bit class, the widths of gate electrodes of CMOS-FETs for memory cell selection and various interconnects are 100 nm or smaller. The amount of current that can be applied to these gate electrodes and various interconnects is limited to about 100 μA or smaller. Therefore, when information is written to a magnetic element by spin-injection magnetization reversal, the writing current needs to be suppressed to about several tens of microamperes. Currently, however, the critical current (writing current) Ic of spin-injection magnetization reversal is on the order of several hundred microamperes to several milliamperes. This large critical current is a bottleneck in practical application of spin-injection magnetization reversal to a non-volatile magnetic memory element array.
The critical current (writing current) Ic of spin-injection magnetization reversal (sometimes referred to simply as the critical current Ic, hereinafter) depends on the time period of writing information (i.e., current pulse width). Typically, the longer the information writing time period is, the smaller the critical current Ic may be. In practical MRAM, it is required that the writing time period be on the order of nanoseconds. Therefore, the following description is based on an assumption that the writing time period is one nanosecond, and the critical current Ic discussed in the following description is premised on this writing time period.
If a magnetic moment fluctuates due to heat, magnetization reversal is possibly caused stochastically. Therefore, the thermal fluctuation resistance directly affects the reliability of MRAM. The thermal fluctuation resistance is expressed as the ratio of the magnetic anisotropy energy of a magnetization reversal layer to the thermal energy thereof, (Ku·V/kB·T). Ku denotes the magnetic anisotropy energy per unit volume of a magnetization reversal layer. V denotes the volume of the magnetization reversal layer. kB denotes the Boltzmann constant. T denotes the absolute temperature of the magnetization reversal layer. Typically, the thermal fluctuation resistance needs to satisfy the relationship (Ku·V/kB·T)>40. The probability of thermal magnetization reversal is expressed by the Boltzmann distribution exp(−Ku·V/kB·T). The time period τ of the occurrence of thermal magnetization reversal is expressed by the equation ln(τ/τ0)=Ku·V/kB·T. τ0 denotes the reversal trial time period (time period of one trial of magnetization reversal), and is about one nanosecond.
The basic configuration of a magnetic element (the structure of a multi-layer film) is based on a magnetoresistance effect film. The magnetoresistance effect film is formed of two magnetic films (a magnetization reversal layer and a magnetization reference layer) that are deposited with the intermediary of a nonmagnetic film therebetween. The magnetoresistance effect film is a multi-layer structure having a GMR effect when the nonmagnetic film is an electrically conductive film, while it is a multi-layer structure having a TMR effect when the nonmagnetic film is an insulating film. The magnetization reversal layer has a low resistance when the magnetization directions of two magnetic layers, in other words, the magnetization reversal layer and the magnetization reference layer, are parallel. In contrast, the magnetization reversal layer has a high resistance when the magnetization directions are antiparallel. This phenomenon (magnetoresistance effect) is used to record magnetic information of “0” or “1” in the magnetic element. Subsequently, an adequate current is passed through the magnetization reversal layer to thereby detect the resistance of the magnetization reversal layer, which allows nondestructive retrieval of the magnetic information recorded in the magnetization reversal layer. In a memory element, it is desirable to use a multilayer structure having a TMR effect, which offers larger outputs.